Some internal combustion engines may operate with one or more cylinders in what may be referred to as a homogeneous charge compression ignition (HCCI) mode, which includes controlled auto-ignition (CAI) of an air and fuel charge via compression performed by a piston of the cylinder. The HCCI mode may be used in some conditions to provide increased fuel economy and/or reduced emissions. As one example approach, a substantially homogeneous mixture of air and fuel within a cylinder is compressed until auto-ignition of the mixture occurs without necessarily requiring an ignition spark to be performed by a sparking device.
In some conditions, an engine operating in an HCCI mode may have a reduced or limited operating range compared with other types of combustion such as spark ignition (SI). In one approach, as set forth in “Thermodynamical and Mechanical Approach Towards a Variable Valve Train for the Controlled Auto Ignition Combustion Process” SAE article 2005-01-0762, two stroke, six stroke, and eight stroke cycles are described for enabling CAI operation in low speed and low load ranges. As one example, a six stroke cycle is described that includes a first combustion event performed with a lean stratified charge via spark ignition and a second combustion event performed with CAI.
However, the inventors of the present disclosure have recognized some issues with the above approach. As one example, when operating an engine in HCCI mode using a charge that is lean of stoichiometry, the effectiveness of some emission control devices such as a NOx catalyst may be reduced. As such, there may be a trade-off between the application of a lean operation for achieving improved fuel economy and catalyst effectiveness which may be reduced during the lean operation.
As such, in one approach, as described herein, the above issues may be addressed by a method of operating an engine having at least one cylinder including an intake valve and an exhaust valve, the method comprising: injecting a first amount of fuel into the cylinder; auto-igniting a first mixture of air and said first amount of fuel by compressing said first mixture; injecting a second amount of fuel into the cylinder after auto-igniting said first mixture; combusting a second mixture of said second amount of fuel and gasses from auto-ignition of said first mixture; holding an intake valve of the cylinder closed between auto-igniting the first mixture and combusting the second mixture; and exhausting said combusted second mixture.
In this manner, one or more cylinders of an engine may be operated where an initial charge of air is used to provide substantially the entire amount of oxygen used by two separate combustion events. As one example, during the first combustion event, a homogeneous charge of air and fuel may be controlled to be lean of stoichiometry and auto-ignited to take advantage of improved fuel economy of lean operation while the second combustion event via auto-ignition of a second homogeneous charge may include a more stoichiometric mixture of air and fuel, thereby enabling improved catalyst effectiveness when exhausted. However, in some conditions, a second combustion event of the cycle may alternatively include spark ignition to provide residual gas of higher temperature that may be used to facilitate subsequent auto-ignition events.
Further, the inventors of the present application have recognized that operating some or all of the cylinders in a six stroke cycle may result in an increased torque imbalance in the engine or vehicle driveline under some conditions. As such, the present disclosure may address this issue by providing a method for varying an operation of at least a second cylinder between a four stroke cycle including a single combustion event and a six stroke cycle including two combustion events based on operating conditions of the engine. In some examples, the operating conditions may include one or more of a performance mode selected by the driver, a level of vibration produced by the engine and/or transmitted to a driveline of the engine, or a condition of the transmission coupled to the engine.